The Milky Way stellar halo is twisted and doubly broken: insights from DESI DR2 Milky Way Survey observation

TL;DR

This study uses DESI DR2 data to reveal the complex, twisted, and doubly broken structure of the Milky Way's stellar halo, showing its shape, orientation, and density variations across different radii and linking features to past accretion events.

Contribution

First detailed measurement of the Milky Way stellar halo's shape, orientation, and density anisotropies using DESI DR2 K giant data, revealing a twisted, doubly broken, and dynamically evolving halo structure.

Findings

The stellar halo is triaxial with axes ratio 10:8:7, tilted 43° from the disk.

Identified two break radii at ~16 kpc and ~76 kpc linked to GSE and LMC.

Outer halo is prolate and perpendicular to the disk, consistent with the Vast Polar Structure.

Abstract

Using K giants from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) Milky Way (MW) Survey, we measure the shape, orientation, radial profile, and density anisotropies of the MW stellar halo over 8 kpc$<r_\mathrm{GC}<200$ kpc. We identify a triaxial stellar halo (axes ratio $10:8:7$), 43 degrees tilted from the disk, showing two break radii at $\sim16$ kpc and $\sim76$ kpc, likely associated with Gaia-Sausage/Enceladus (GSE) and Large Magellanic Cloud (LMC), respectively. The inner stellar halo ($<30$ kpc) is oblate and aligned with the disk, whereas the outer stellar halo becomes prolate and perpendicular to the disk, consistent with the Vast Polar Structure of MW satellites. The twisted halo may arise from the disk-halo angular momentum shift triggered by the infall of a massive satellite. The anisotropic density distribution of the stellar halo is also measured, with successful re-identification of the Hercules-Aquila Cloud South/North (HAC-N/-S) and Virgo overdensities (VOD). Break radii are found at 15/30 kpc for VOD/HAC-N(-S). We identify the LMC transient density wake with a break radius at 60 kpc in the Pisces overdensity region. We also find new observational evidence of the LMC collective density wake, by showing a break radius at $\sim$100 kpc in the northern Galactic cap with a clear density peak at 90 kpc. In the end, we found that more metal-poor halo stars are more radially extended. Our results provide important clues to the assembly and evolution of the MW stellar halo under the standard cosmic structure formation framework.